sr: Eliminate usb/serial instances from API.

sr: Eliminate usb/serial instances from API.

The API should be generic and only provide sr_device_instance_new() and
friends, but not sr_usb_device_instance_new(), sr_serial_device_instance_new(),
or others for other device types we may have in the future. The
frontends shouldn't have to know or care about this implementation detail.

This also fixes the problem that parts of sigrok.h contained
'#ifdef HAVE_LIBUSB_1_0' and such, which is even less desirable for the API.

The usb/serial instance specifics are now private, and each driver that
needs them keeps a pointer in its driver-specific context.